X-ray tube



March 29, 1938. Y A. WOLFEL Y 2,112,746

X-RAY TUBE Filed April 16, 1935 Inventor:

ttorney.

Albrecht Wdlfel Patented Mar. 29, 1938 UNITED STATES X-RAY TUBE AlbrechtWolfe], Rudolstadt, Germany, assignor to General Electric Company, acorporation of New York Application April 16, 1935, Serial No. 16,673 InGermany May 15, 1934 1 Claim.

In order to adapt an X-ray tube to more than one purpose, for example,therapeutic and radiographic, which necessitate different loadconditions, i. e. different sizes of focal spots, etc., one may providethe tube with two or more filamentary cathodes of difierentelectron-emitting capacities. Obviously, the cathodes require differentamounts of heat. Consequently, when changing the operation of the tubefrom one use to the other, as for example, when substituting the cathodeof smaller electron-emitting or heating capacity for the larger cathode,and assuming that it is desired toenergize the smaller cathode from thesame source of electromotive force as r the larger cathode, it isdesirable to reduce the heating voltage impressed across the smallercathode.

In order automatically to reduce the energy supply to the smallerfilament under these conditions, it has' been proposed to insert aresistance in the heating current circuit of the oathode and in case thecathode is energized by an alternating current source, a choke coil or atransformer may be used. The choke coil, transformer or resistance isusually connected in the filament circuit external to the tube and inorder to make the structure of a unitary design, it has been proposed toenclose the coil or resistance in the base cap of the tube. However,when the voltage-reducing device is enclosed within the base cap,certain difiiculties arise. In the first place, the cap on the cathodeside of the tube must be enlarged to accommodate the extra element whichmay present some difficulty in fitting the tube and its cap intoexisting apparatus and protective housings. In addition, more especiallywhen using a resistance, but also in the case of a choke coil ortransformer in lesser degree, there is the further disadvantage in thatthe voltage-changing device may give off considerable heat anddeleteriously affect the material of the cap.

An object of the present invention is to provide an improved all-purposeX-ray tube containing two or more filamentary cathodes of differentelectron-emitting or heating capacities, and in which one cathode may besubstituted for the other cathode when desired, and is adapted to beenergized from a common source of electromotive force. A more specificobject is to em.- ploy in a tube of this character, an improved form ofvoltage-reducing device or element in connection with the cathode ofsmaller electronemitting or heating capacity and also an improvedposition of the device. In carrying out these objects, my inventioncontemplates in brief, the use of a resistance or inductance in serieswith the smaller filament, i. e. the cathode of lower heating capacity,and mounting the resistance or inductance within the confines of theenvelope so as to be surrounded by an evacuated space in case theenvelope is highly evacuated. The invention will be clearly understoodwhen reference is made to the following description and the accompanyingdrawing in which Fig. 1 represents a view, partly in section, of anX-ray tube containing a cathode construction improved in accordance withthe present invention; Fig. 2 is an end view of the focusing member andassociated cathodes; while Fig. 3 is an enlarged diagrammatic view ofthe cathode structure per se including the voltage-reducing element.

Referring more particularly to Fig. l, numeral l designates the envelopeof an electrical discharge device containing an anode 2, which in thecase of an X-ray tube may have an inclined face and be provided with atarget. The anode is supported Within the envelope in any suitablemanner, for example, by means of a rigid rod 3 secured to a reentrantstem 4 which terminates one end of the envelope. The other end of theenvelope is also provided with a reentrant stem 5 in which are fusedseveral rigid rods serving as combined leading-in conductors andsupports for the improved cathode structure. This structure includesfilaments 6, 1, one of the filaments,

e. g. filament 6, being of greater heating capacity and therefore ofgreater electron-emitting capacity than the other filament. There mayalso be provided a hemispherical focusing member 8, supported within theenvelope by means of a rod 9 which is secured to one of the conductorspassing through the stem 5. The conductor 9 may be connected to one orboth of the filamen tary cathodes or as is more usually the case, may betaken out through the envelope as a separate lead to which any desiredpotential may be applied.

The cathodes are preferably constituted of the same material, forexample, tungsten wire of the same gage, and since one cathode isintended to emit greater amounts of electrons than the other cathode, itfollows that the lengths of the respective cathodes are different.

When a tube of this character is operated, it is desirable to be able toswitch from one cathode to the other in case the same tube isalternatively serving two Widely different purposes, as for example,when used in therapy and radiography. In case a common source ofelectromotive force is employed to energize both cathodes, obviously thevoltage impressed across the cathode of smaller heating orelectron-emitting capacity must be less than the voltage applied to thecathode of larger heating or electron-emitting capacity. The necessaryreduction of voltage in case it is desired to substitute the smallercathode for the larger cathode, is conveniently obtained by means of aresistance or choke coil l connected in series with the smallerfilament 1. As shown, the adjacent ends of the filaments 6, I areconnected together by a common leading-in conductor and the opposite endof the larger filament is connected to another leading-in conductor,while the free end of the element I0 is connected to the remainingleading-in conductor. The filaments are conveniently arranged side byside within the opening at the bottom of the spherical focusing member 8as is shown more clearly in Fig. 2. When the filaments 6, l areenergized from a common source I I of alternating or direct current, andwhen a switch I2 is provided in the circuit for selectively connectingthe filament 6 or I. to the source, the same voltage may now be applied,without danger to the smaller filament 1, as isapplied to the largerfilament 6. Obviously, the voltage actually impressed across thefilament 1 represents a reduced voltage obtained by the effect of theresistance or inductance l0 which serves to dissipate or consume theenergy of the undesired voltage. It is desirable when the element i0 isconstituted of resistance, that the latter should be made of the samewire as the filaments. 6 and I. It is also apparent that the combinedresistance of the filament 'l and the resistance II] should besubstantially equal to the resistance 'of the filament 6. By placing thevoltage-reducing element I0 within the envelope instead of beingpositioned external to the envelope and contained within a base cap atthe cathode end of the tube, the element is in such a position as not tocause any deleterious heating efi'ects, nor does it necessitate a largecathode cap within which to be accommodated. It will be advantageous tomount theresistance close to the cathode, since in this case the heatgenerated in it will heat only such internal constructional parts of thetube as are heated by the cathode itself and are therefore designed towithstand the heat produced. Moreover, the improved position of theelement I0 ofiers a unitary and compact construction.

In the event that the element I0 is required to dissipate or consumeconsiderable energy and thereby is rendered sufiiciently incandescent toemit electrons, it may be desirable to provide a metal shield 13interposed between the element and the anode. This shield may be givenany suitable shape, e. g. an angular configuration in order to provide amechanical barrier for electrons which tend to leave the end of theelement Ill near the envelope. The shield may be supported within theenvelope in any suitable manner.

In addition to the shield I3 and as shown in Fig. 3, there may beprovided other angular members I4 positioned respectively between thefree ends of the filaments 6, l and the envelope. All of these shieldsor screens serve to prevent any electrons from reaching the anode fromundesired parts of either cathode or associated elements.

While I have described my invention more especially in connection withan X-ray tube for the reason that cathodes of differentelectron-emitting capacity are frequently employed in. this connection,it will be understood that the present invention is not limited to tubesof this type but is sufiiciently broad to embrace all forms ofelectrical discharge apparatus, highly evacuated or gas-filled, in whicha plurality of filamentary cathodes are employed having differentelectrical characteristics and which are selectively energized from acommon source of electrom'otive force.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is,

An electrical discharge device comprising an envelope containing ananode, a main cathode and a secondary cathode, said cathodes beingarranged adjacent to one another, a voltage-reducingelement within saidenvelope and connected to said secondary cathode, the combinedresistance of the secondary cathode and said element being substantiallyequal to the resistance of said main cathode whereby equal voltages maybe applied to the main and secondary cathode circuits to produce equalheating currents, and a shield mounted adjacent to said voltage-reducingelement for preventing radiation of electrons by said element butpermitting radiation by the secondary cathode.

ALBRECHT WOLFEL.

